The phrase "stick fast" means to adhere strongly or tightly. Therefore, some antonyms for this phrase could include "loose," "detachable," "unfastened," and "unfixed." Other possible antonyms might be "dislodged," "separated," "unattached," or "unconnected." Additionally, words like "slippery," "smooth," "slick," or "greasy" could be used to describe the opposite of "stick fast," as these terms imply a lack of adhesion or resistance to movement. Ultimately, the choice of antonyms will depend on the context of the sentence and the specific connotations or shades of meaning that are desired by the speaker or writer.